Undulating, slow wave structure for an electron discharge device



Nov. 26, 1968 BUCK 3,413,512

D. UNDULATING SLOW WAVE STRUCTURE FOR AN ELECTRON DISCHARGE DEVICE Filed 001., 20, 1965 2 Sheets-Sheet l Nov. 26, 1968 BUCK UNDULATING SLOW WAVE STRUCTURE FOR AN ELECTRON DISCHARGE DEVICE Filed Oct. 20, 1965 '2 Sheets-Sheet 2 u w 4 2 2 G A: H a. Q 6 42 2 5 4 Q. 2 m? a\ 2 9 o \2 9 0 w 8 m o w m F f f \A w 2 2 O 6 5 6 "a 6 7 3 2 "a M "a \m 2 5 2 2 W I 'l "a a M 8 3 United States Patent 3,413,512 UNDULATING, SLOW WAVE STRUCTURE FOR AN ELECTRON DISCHARGE DEVICE Daniel C. Buck, Horseheads, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 2!), 1965, Ser. No. 498,919 1 Claim. (Cl. SIS-3.5)

ABSTRACT OF THE DISCLOSURE This disclosure relates to a slow wave structure including at least first and second electrically conductive members having an undulating or substantially sinusoidal configuration. Corresponding points of maximum excursion of the first and second electrically conductive members are connected by transverse electrically conductive bars to form the electrical equivalent of at least two counterwound helixes. Further, this disclosure describes a slow wave structure and a method of forming this structure in which first and second substantially sinusoidal conductive members are disposed in a surface and are connected at their points of maximum excursion. Then the surface is disposed in an undulating or sinusoidal configuration to provide a slow wave structure which is the equivalent of at least two counter-wound helixes. In a second method, the surface is formed about a mandrel and the other set of points of maximum excursion are electrically connected to form a slow wave structure.

The present invention relates generally to electron discharge devices and more particularly to slow-wave or delay line propagation structures for use in circuits or devices such as traveling wave tubes.

Traveling wave tubes comprise generally an elongated envelope with an electron beam producing means disposed at one end thereof for the production and projection of an electron beamthrough the envelope. A slow wave propagating means coaxially mounted within the envelope functions to propagate electromagnetic energy along the length of the envelope in an interacting relationship with the electron beam. An electron collector assembly is disposed at the opposite end of the envelope for collecting the electron beam. In operation, an electromagnetic wave is first established upon the slow-wave structure whereby the wave is propagated along the slow-wave structure at a velocity substantially less than the velocity of light. The electron beam is then projected through the slow-wave structure at approximately the same velocity as that of the electromagnetic wave. Mutual interaction of the electron beam and the electromagnetic wave affects a transfer of energy from the electron beam to the wave causing the electromagnetic wave to grow or to be amplified.

Typically, a slow-wave structure comprises a helically shaped electrically conductive member which supports the traveling electromagnetic wave as described above. The helically shaped member provides good traveling wave tube performance for tubes in which the electron beam has been accelerated to velocities less than about kv., or about one fifth the velocity of light. For tubes requiring output power over about one kilowatt at 10 cycles per second, the required acceleration voltage typically exceeds 10 kv. A helically shaped member designed for operation above 10 kv. or an axial phase velocity of greater than one fifth the velocity of light, tends to provide unstable interaction With the electron beam. In particular, the spacing between turns of the helically shaped member is large compared to the diameter of the wire of which the helically shaped member is wound. This unstable interaction can be best understood by considering the spatial harmonics of the electromagnetic field distribution on the helically shaped member. Since the interaction circuit, in this case a helically shaped member, is a spatially periodic structure, the electromagnetic fields associated with any frequency of excitation can be represented by a series of spatial harmonics, whose wavelengths as measured along the axis of the interaction circuit are submultiples of the fundamental wavelength of the interaction circuit. Each of these component spatial harmonic field configurations moves with a different phase velocity along the axis of the helically shaped member. In general, the higher the order of submultiple of the fundamental wavelength, the slower is its corresponding axial phase velocity.

In a traveling wave tube of the helix type, the shapes and relative sizes of this interaction circuit is adjusted to emphasize the fundamental space harmonic at the expense of the other space harmonics. In the operation of a helically shaped member above 10 kv., the turn-to-turn spacing becomes substantially larger than the diameter of helix wire such that the fundamental space harmonic can no longer be made large compared to the next higher order space harmonic. This next higher space harmonic tends to make the tube operate as a backward wave oscillator, and therefore the tube is unsuitable as an amplifier.

It has been shown that a noncontacting cross-wound, slow-wave structure having two helices which are oppositely wound and interposed has a mode of operation which possesses a higher impedance in the fundamental harmonic and lower impedance in the backward wave spatial harmonic than the single helix for voltages greater than 10 kv. (see: Chodorow and Chu, Cross-Wound Twin Helices for Traveling-Wave Tubes, Journal of Applied Physics, vol. 26, pp. 3343, January 1955). It has also been shown that such a cross-wound structure has less velocity dispersion than the usual ring-and-bar structure which is in general present day use.

However, such a non-contacting structure in which the two helices have the same radius and longitudinal axis and which is suitable for high frequency operation has not lent itself to being readily manufactured. Typically, it has been necessary to perform an expensive machining operation in order to provide slots of precise dimensions within a unitary cylindrical body. Further, the formation of a slowwave structure from tubing which could be readily cooled as by directing a fluid therethrough, has been performed by winding the tubing into complicated, helical configurations.

It is therefore an object of this invention to provide an improved electron discharge device.

A further object of this invention is to provide an improved slow-wave propagating means for use in electron discharge devices.

A further object of this invention is to provide a slowwave propagating structure which may be easily manufactured and which avoids either expensive milling operations or the winding of helical structures.

A still further object is the provision of a slow-wave propagating structure which may be easily cooled by the flow of a cooling medium therethrough.

Briefly, the present invention accomplishes the abovementioned objects by providing a traveling wave tube which utilizes a slow-wave structure including undulated, electrically conductive members having a plurality of peaks which are electrically connected to provide the electrical equivalent of counter-wound helics. Further, in one specific embodiment of this invention, the electrically conductive members are made of a tubing through which a cooling medium may be directed to dissipate the heat generated by the slow-wave structure of this invention.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claim annexed to and forming a part of the specification.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

FIGURE 1 is an elevational view partially in section, of a traveling wave tube embodying the present invention;

FIG. 2 shows an intermediate step of manufacturing the slow-wave structure of this invention;

FIGS. 3a and 312 show a further step in the process of manufacturing embodiments of this invention whose initial step of manufacturing was shown in FIG. 2;

FIG. 4 is an isometric view of a completed slow-wave structure, whose method of manufacture was shown in FIGS. 2 and 3b and which may be incorporated in the electron discharge device of FIG. 1.

FIG. 5 is an isometric view of an additional embodiment of this invention which may be incorporated in the electron discharge device of FIG. 1;

FIGS. 6 and '7 are cross-sectional views of modifications of the slow-wave structure of FIG. 5 which may be incorporated within the electron discharge device of FIG- URE 1; and

FIGS. 8 and 9 are additional modifications of the slowwave structure of FIG. 5 in accordance with the teachings of this invention.

Referring now to the drawings and in particular to FIG. 1, there is shown an electron discharge device 10 of the traveling wave tube type comprising an envelope 12 including a central tubular portion 14, which may be made of a suitable material such as glass, and an enlarged portion 16 which is sealed to the tubular portion 14. An electron beam source, indicated generally by the reference character 20, is disposed at one end of the envelope 12 within the enlarged end portion 16. The beam source 29 comprises a thermionic electron emissive cathode element 22, a heater element 23 to energize the cathode element 22, a focusing electrode 24, and an accelerating electrode 26. These elements are connected to suitable sources of voltage, which have not been shown, and collectively act to produce and direct a beam of electrons centrally along the axial length of the envelope 12 to the opposite end of the envelope. An electron collector 28 is positioned at the opposite end of the envelope 12 and serves to collect the electron beam after it passes through the tubular portion 14.

A slow-wave propagating means or structure 36 is posi tioned intermediate of the electron beam source 26 and the electron collector 28. The propagating structure 36 functions to propagate electromagnetic energy along the length of the tubular portion 14 in an interacting relation ship with a major portion of the outer edge of the electron beam. The propagating structure 36, in accordance with the teachings of this invention, includes two undulating or sinusoidally-shaped, electrically conductive members which are formed into an effective cross-wound helix as will be more fully explained later.

Electromagnetic energy is supplied to the slow-wave propagating structure 36 by means of a wave guide 36 which is positioned near the enlarged end portion 16 about a ferrule 33 in a manner well known in the art. Similarly, a second wave guide structure 32 is provided at the opposite end of the tubular portion 14 about a ferrule 35 for the removal of electromagnetic energy from the slow-wave propagating structure 36.

The propagating structure 36 is supported within the envelope 12 by means of a plurality of supporting members 38 which are disposed along the axial length of the tubular portion 14 for substantially the entire length of the propagating structure 36. The support rods are preferably made of a suitable heat conducting, electrical insulating material such as beryllia.

In order to prevent the electron beam from spreading to such an extent that it would be intercepted by propagating means 36, it is necessary to provide some form of focusing. Focusing in this illustrative embodiment is provided by producing a magnetic field axially along the envelope 12 by utilization of a long, annular solenoid 34 which surrounds the tubular portion 14 of the envelope 12 for substantially the entire length of the propagating structure 36. To simplify the present drawing and description, the solenoid 34 is only schematically illustrated and its source of energization is not shown.

With reference now to FIGS. 2 and 3a, an exemplary embodiment of the slow-wave propagating structure 36 will be shown. The slow-wave propagating structure 36 includes a first and second undulated or sinusoidallyshaped electrically conductive members 52 and 54 having a plurality of peaks or points of excursion which are connected at the joints designated 60 to 63. In the first step of the assembling process as shown in FIG. 2, the two electrical conductive members 52 and 54 are shaped upon a jig 56. The jig 56 includes a platform 57 upon which are mounted a plurality of pegs 58 arranged so that a peg is disposed at the peak point or point of maximum excursion of the sinusoidal configurations in which the conductive members 52 and 54 are to be shaped. Illustratively, two lengths of the electrically conductive member are bent about the pegs 58 in order to form the two conductive members 52 and 54 having a series of maximum points of excursion which are in physical contact with each other and which may be brazed in a manner well known in the art to form the joints 60 to 63. Further, as shown in FIG. 2, the electrically conductive member 52 has a series of maximum points of excursion designated by the numerals 60, 67, 61, 68, 62, 69 and 63; the electrically conductive member 54 has a series of points of maximum excursion designated by the numerals 60, 64, 61, 65, 62, 66 and 63. Typically, the points of maximum excursion of the conductive member 52 are successively spaced from each other by distances corresponding to the distances the successive points of maximum excursion of the conductive member 54 are spaced from each other.

Referring now to FIG. 3a, the second step in forming the slow-wave propagating structure 36 is shown. In this step, the slow-wave propagating structure 36 is removed from the jig 56 and is bent to conform to the configuration as established by a second jig 70. The second jig 70 comprises a platform 71 and a plurality of pegs 72 disposed at the maximum points or peaks of a sinusoid to which shape the planar wave structure 36 is to be shaped. As shown in FIG. 3a, the first and second electric conductive members 52 and 56 are bent to conform to a surface 75 defined by a pair of lines 73 and 74 (shown as dotted lines). In particular, the joints 60, 61, 62 and 66 are established as the points of maximum excursion of the sinusoidal surface 75. Thus, it may be understood that there has been formed a double, counter-wound, slow-wave helical structure in accordance with the teachings of this invention more specifically, the first helix of the slow-wave propagating structure 36 may be realized by tracing the numerals in the following order: 60, 64, 61, 68, 62, 66 and 63. The second helix of the slow-wave propagating structure 36 may be perceived by following the reference numerals in the order enumerated: 66, 67, 61, 65, 62, 69 and 63. Further, it may be visualized that there are formed a plurality of loops by the conductive members 52 and 54 through which the electron beam emitted by the source 20 is directed.

Referring now to FIG. 3b, there is shown the second step in forming another embodiment of a slow-wave structure in accordance with the teachings of this invention. The first step in forming a slow-wave propagating structure 136 has been shown and explained with regard to FIG. 2. Next, the slow-wave propagating structure 136 would be removed from the jig 56 and disposed about a mandrel as shown in FIG. 3b. More specifically, the mandrel 170 has been illustrated as a round, tubular form; it is noted that the mandrel 170 could be of any suitable configuration corresponding to that of the envelope 12. As explained before, the slow-wave, propagating structure when taken from the jig 56 is comprised of first and second electrically conductive members 152 and 154, each shaped sinusoidally and disposed in a surface 172. In the second step of forming, the surface 172 of the slow-wave propagating structure 136 is bent or formed about the mandrel 170 so that the points of maximum excursion of the electrically conductive members 154 and 152 are brought into contact at the points designated by the numerals 164, 165 and 166. After the slow wave propagating structure 136 has been so formed, the points of maximum excursion of the members 152 and 154 are secured as by brazing to form permanent joints at the points designated by the numerals 164, 165 and 166. Referring now to FIG. 4, a finished, double wound propagating structure 136 is shown. Specifically, the sinusoidally-shaped conductive member 152 is secured to the second electrically conductive member 154 at the joints 160, 161, 162, 163, 164, 165 and 166. It is noted that each of the electrically conductive members 152 and 154 is sinusoidally shaped and is in addition disposed in the enclosed surface 172 about the electron beam emitted from the source 20. Thus, it may be understood that the first helix of the double, counter-wound helix is formed by the structure 136 as designated by the following portions and numerals: the portion of the conductive member 154 between the numerals 160 and 164; the portion of the conductive member 152 between the numerals 164 and 161; the portion of the conductive member 154 between the numerals 161 and 165; the portion of the conductive member 152 between the numerals 165 and 162; the portion of the conductive member 154 between the numerals 162 and 166; and the portion of the conductive member 152 between the numerals 166 and 163. The second helix of the counter-wound, slow-wave propagating structure 136 may be traced by the following portions and numerals: the portion of the conductive member 152 between the numerals 160 and 164; the portion of the conductive member 154 between the numerals 164 and 161; the portion of the conductive member 152 between the numerals 161 and 165; the portion of the conductive member 154 between the numerals 165 and 162; the portion of the conductive member 152 between the numerals 162 and 166; and the portion of the conductive member 154 between the numerals 166 and 163.

Referring now to FIG. 5, an additional embodiment of the slow-wave propagating structure identified by numeral 236 is shown. Specifically, the slow-wave propagating structure 236 consists of first and second electrically conductive members 252 and 254, each of which is shaped sinusoidally. The first conductive member 252 has a plurality of peaks or points of maximum excursion identified by the numerals 277 to 283 and the second conductive member 254 has a series of peaks identified by the numerals 270 to 276. Further, the peaks or points of maximum excursion of the first and second conductive members 252 and 254 are connected to a plurality of conductive bars or support members 256 to 262. As shown in FIG. 5, the peak 277 of the first conductive member 252 is secured as by brazing to the bar 256 which is also connected as by brazing to the peak 270 of the conductive member 254. The bars 257 to 262 are likewise connected to the corresponding peaks or points of maximum excursion of the conductive members 252 and 254 in a manner as described above. It may be understood that the first helix of the slow-wave propagating structure 236 is formed by the structure shown in FIG. 5 as outlined by the following numerals: 270, 271, 278, 279, 272, 273, 280, 281, 274, 275, 282 and 283. The second helix is formed by the structure traced by the following numerals: 277, 278, 271, 272, 279, 280, 273, 274, 281, 282, 275 and 276. Further, it may be visualized that the slow-wave structure 236 forms a plurality of enclosed loops which surround the electron beam directed axially through envelope 12. In a particular embodiment of this invention, one of the loops is formed by the bars 256 and 260, and the portions of the conductive members 252 and 254 between the reference numerals 277 and 278, and 270 and 271.

Referring now to FIGS. 6 and 7, there are shown illustrative embodiments of the slow-wave propagating structure 236 and how it may be mounted within the envelope 12. Referring specifically to FIG. 6, the first and second conductive members 252 and 254 respectively are supported upon and are connected to (see also FIG. 5) a plurality of straps or support members designated by the numerals 260a and 256a. The strap 260a has flange portions 290 disposed at either end thereof which are secured as by brazing to the support members 38. Likewise, the strap 256a has similar twisted, flange portions 291 disposed at either end thereof and secured in like manner to the support members 38. Further, each of the support members 38 are disposed against the interior surface against the tubular portion 14 of the envelope 12 to thereby firmly position the slow-Wave propagating structure 236. Referring now to FIG. 7, the slow-wave propagating structure 236 is disposed within the envelope 12 in a manner similar to that shown in FIG. 6. A modification has been made in FIG. 7 to the extent that unitary, insulating support members 38a are secured as by brazing between the flange portions 290 of the support member 260a and the flange portions 291 of the support member 256a. Further portions of each of the support members 38a are disposed in an abutting relation against the interior surface of the portion 14 of the envelope 12 so as to firmly position the slow-Wave propagating structure 236.

Referring now to FIGS. 8 and 9, a further embodiment of this invention is shown incorporating a rigid structure for mounting the slow-wave propagating structure 236. Referring specifically to FIG. 8, the first and second conductive members 252 andl 254 respectively are connected at their peaks or points of maximum excursion to a pair of stubs or support members 293 and 294 as by inert gas arc welding. The stubs 293 and 294 are made of a suitable electrically conductive material such as a nonmagnetic stainless steel 304 and have tab portions 295 which are secured as by inert gas are welding to the interior surface of a portion 14a of an envelope 12a. It is noted that in this additional embodiment, that the envelope 12a may be made of a suitable electrically conductive material such as the above-mentioned stainless steel. In the additional embodiment shown in FIG. 8, the stubs 293 and 294 extend across and are secured to the opposite sides of the envelope 12a. It is especially noted that the first and second conductive members 252 and 254 are secured to the stubs 293 and 294 so as to leave a spacing between the joint at which the conductive members are secured thereto and the interior surface of the envelope 12a by a length equal to /4 of the median, operating wavelength of the traveling wave tube 10. By so determining this distance, the stubs 293 and 294 will appear to the electromagnetic wave established upon the slow-wave propagating structure 236 as an open circuit.

Referring now to FIG. 9, single stubs 296 and 297 are respectively connected to the maximum points of excursion of the first and second conductive members 252 and 254 respectively. Further, the single stubs 296 and 297 are connected to opposite walls of the envelope 12a. The point at which the conductive members are secured upon the stubs is spaced from the walls: of envelope 12a by a distance equal to A the median operating, wavelength of the traveling wave tube 10.

The first and second conductive members which are shaped sinusoidally and constitute the slow-wave propagating structure are made of a suitable electrically conductive material such as a molybdenum or the nonmagnetic stainless steel 304, Further, the conductive members may be made of a wire or conduit of a tubular configuration. It is a significant aspect of this invention that a tubular means may be used to construct the first and/or second conductive members so that a liquid coolant may be used to cool the slow-wave propagating structure. In one specific embodiment of this invention, a stainless steel tubing was used with an outer diameter of approximately .010 inch and an inner diameter of .006 inch to construct the first and second conductive members. An exemplary cooling means is illustrated in FIG. 1 in which both of the conductive members are shown to be connected at one end to an input conduit 42 into which fluid is supplied. At the other end of the slow-wave propagating structure 36, the two conductive members are connected to an output conduit 46 for removing the cooling fiuid from the slow-wave propagating structure 36. The conduits 42 and 46 may in turn be connected to a fluid circulating means such as a pump 43.

Thus, there has been shown a slow-wave propagating structure for a traveling wave tube having a configuration which is particularly adaptable to being liquid cooled and which may be easily formed without the expensive processing of machining or forming helical windings. More specifically, there is shown a slow-wave propagating structure having at least two sinusoidally-shaped, conductive members which are connected at their points of maximum excursion to form a unitary structure having the efiective characteristic of at least two counter-wound helices. It is noted that this structure may be formed to have the characteristics of higher even order helices.

While the slow-wave structure of this invention has been described with respect to its use in a traveling wave tube, such a structure could be used in microwave delay devices such as delay lines or filters, and as an antenna for radiating or receiving wave energy.

While there have been shown and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore,

that the invention be limited to the specific arrangement shown and described and it is intended to cover in the appended claim all such modifications as fall within the true spirit and scope of the invention.

I claim:

1. An electron discharge device comprising an envelope; means disposed at one end of said envelope for the production and projection of an electron beam through said envelope; means disposed at the opposite end of said envelope for collecting said electron beam; and slow-wave propagating means disposed within said envelope in an interacting relationship with said electron beam; said slow-wave propagating means comprising first and second electrically conductive members each being shaped in an undulating configuration and having a plurality of peaks, and a plurality of electrically conductive bars, the corresponding peaks of said first and second electrically conductive members being interconnected by at least one of said plurality of electrically conductiye bars to thereby form the electrical equivalent of at least two counterwound helices, said electrically conductive bars being secured to said envelope, said electrically conductive bars being connected to said envelope at a distance equal to one fourth of the median operating wavelength of said electron discharge device from the point of connection of said electrically conductive member to said electrically conductive bar.

References Cited UNITED STATES PATENTS 2,957,103 10/1960 Birdsall 315-36 3,089,975 5/1963 Washburn 3l539.3 X 3,273,081 9/1966 ItZkan 3l539.3 X

HERMAN KARL SAALBACH, Primary Examiner.

S. CHATMON, JR., Assistant Examiner. 

